1. Field of the Invention
The present invention relates to a sheet processing apparatus which processes bundled and folded sheets.
2. Description of the Related Art
A toner image is formed on a sheet at an image forming portion and then the toner image-formed sheet is processed. In such a sheet processing apparatus, a process of binding a bundle of sheets, for example, at near the center of the conveying direction with staples and folding it in half from the bound part to have a booklet shape and discharging it, a so-called saddle stitch binding operation, has been widely performed. Further, various methods which improve the appearance and quality of the booklet by evenly cutting the edges of the bundle of the sheets folded in half with a cutting apparatus and performing trimming have been proposed (refer to Japanese Patent Application Laid-Open (JP-A) No. 2000-198613).
Incidentally, there are the following problems to be solved in the conventional sheet post-processing apparatus including a sheet cutting method and a sheet cutting apparatus, described in JP-A No. 2000-198613. They will be briefly described with reference to FIGS. 15A to 15E.
An operational sequence in which a plurality of sheets is bundled and bound with staples in the central part along the conveying direction and then the edges of the sheet bundle T folded in half and discharged are cut, which are subjected to trimming will be illustrated with reference to FIGS. 15 (a) to (e).
At the stage before the sheet bundle T is conveyed to a trimmer, a width direction of the sheet bundle folded in half is regulated by side guides during the conveyance to reach the trimmer and the skew feeding is corrected. The sheet bundle is carried on conveying belts, which rotate to convey the bundle.
At this point, it is necessary to enhance a poor crease line of the sheet bundle T by pressing the crease with pressing members so that a space between the edges of the sheet bundle is not opened. This is because the space between the edges is opened as shown in FIG. 15B even when it is folded in half as shown in FIG. 15A. However, smooth conveyance of the sheet bundle T is interrupted by the resistance of the pressing force exerted by pressing guides. In some cases, the sheet bundle may not be conveyed. Further, when a color image is recorded on the top of the sheet bundle T, or alternatively smooth-coated paper or a thin paper sheet is used, the coefficient of friction between the conveying belts and the front cover of the sheet bundle T is low. Therefore, the sheet bundle T sometimes will not be conveyed due to its sliding. In order to support such a conveyance, a structure in which a claw-shaped pushing member that operates at the same speed as the running speed of the conveying belts pushes the rear end of the sheet bundle T from behind is also proposed.
However, because the claw-shaped pushing member operates at the same speed as the running speed of the conveying belts, the claw-shaped pushing member cannot surely push the rear end of the sheet bundle. Therefore even if it has such a pressing structure, only the front cover sheet of the sheet bundle T may be advanced due to the contact friction with the conveying belts as illustrated in FIG. 15C.
If the edges of the sheet bundle is cut in the state that the top of the front cover sheet of the sheet bundle T is advanced, the edges of the front cover sheet are protruded from the edges of the middle sheet as illustrated in a circle in FIG. 15D, resulting in a booklet with very bad appearance. As illustrated in FIG. 15E, it is desired that the edges of the sheet bundle T having a booklet shape are cut finely and evenly.
As an example of the problems, the problem that the edges of the front cover sheet of the sheet bundle after cutting are not evenly aligned with the edges of other sheets and they are protruded from the edges of the sheet bundle is listed. However, as illustrated in FIG. 15C, when the booklet is not subjected to the cutting process, its cover sheet is misaligned, which looks bad. Further, in the case where the sheet bundle is bound at the folded portion, as illustrated in FIG. 15C, when only the top of the front cover sheet is advanced, a large load acts on the bound portion with staples of the front cover sheet. At the part close to the bound portion with staples, breakage in which the front cover sheet is torn may occur.
When the sheet bundle is conveyed by the belts, the state of the sheet bundle becomes like that of FIG. 15C. This misalignment becomes significant in the case where the contact friction between the surface of the conveying belts and the front cover sheet of the sheet bundle is larger than the coefficient of friction of the inner surface of the front cover sheet and the middle sheet. That is, for example, in the case where a monochrome image is recorded on the front cover sheet and a color image is recorded on the inner surface of the front cover sheet and the middle sheet, respectively, the failure in which only the front cover sheet is advanced frequently occurs.